1. Field of the Invention
The present invention relates to a dry etching method and, more particularly, to a method of dry-etching a film to be etched containing a plurality of materials having different etching rates.
2. Description of the Related Art
In recent years, a large-scale integrated circuit (LSI) formed by connecting a large number of transistors, resistors, and the like to constitute electrical circuits and integrating the electrical circuits on one chip is popularly used in an important portion of a computer or a communication machine. For this reason, the performance of the overall machine is largely dependent on the performance of each LSI.
The performance of each LSI can be improved by increasing the degree of integration, i.e., micropatterning an element. For this reason, a tendency toward further micropatterning of an element continues, and a highly precise pattern size is strongly demanded.
A semiconductor integrated circuit is generally formed by stacking an insulating thin film such as a silicon oxide thin film having a predetermined pattern, a conductive thin film of, e.g., aluminum, and the like on a semiconductor substrate such as a silicon substrate.
As techniques for processing these thin films in a predetermined pattern, the following techniques are conventionally used: a lithographic technique in which after a photosensitive photoresist is coated on a thin film, the photoresist is exposed in a desired pattern by a light or ultraviolet ray, and an exposed portion or an unexposed portion of the photoresist is selectively removed by developing, thereby forming a pattern in the photoresist; a dry etching technique for etching an underlying thin film using the photoresist pattern as a mask; and a peeling technique for removing the photoresist.
At the present, as one method of processing an underlying thin film in a desired pattern using a fine photoresist pattern, a reactive ion etching (RIE: Reactive Ion Etching) technique using a plasma is popularly used. According to this method, a substrate on which a film to be processed is deposited is loaded in a vacuum vessel having a pair of parallel-plate electrodes, the vacuum vessel is evacuated, and a reactive gas containing a halogen element or the like is supplied into the vacuum vessel. In addition, the reactive gas is transformed into a plasma by discharge obtained by applying RF power, and the film to be processed is etched by the generated plasma.
According to the method, since ions among various particles in the plasma are accelerated by a DC electric field generated by an ion sheath on the electrode surface, the ions having high energy collide against the film to be etched so as to cause an ion-accelerated chemical reaction. For this reason, etching progresses in the ion incident direction, and processing having excellent directivity and being free from an undercut can be achieved.
However, it has been found out that a method of this type has the following drawbacks.
At present, an Al alloy obtained by adding a small amount of Si or Cu to Al is used as the material of Al wiring layers. Cu in the Al alloy cannot be easily removed because the vapor pressure of a compound of Cu and a halogen serving as an etching gas is low, and after the Al thin film is etched, etching residues are formed due to the presence of Cu. These etching residues degrade the insulating property between wiring layers, and pose a problem such as corrosion, thereby remarkably degrading reliability of the wiring layers.
Although formation of the etching residues can be suppressed by increasing ion energy in the plasma etching, in this case, the etching selection ratio with respect to the resist ((etching rate of Al thin film)/(etching rate of photoresist)) is considerably low, and patterning at a high precision cannot be performed.
In order to solve this problem, the following method is used. That is, in etching an Al thin film, a multilayered resist is formed on the Al thin film, a pattern having a high aspect ratio is formed on the multilayered resist, and the Al thin film is etched using the pattern as a mask.
For example, when a three-layered resist process is to be used, after an Al film serving as a film to be processed is deposited on a substrate, an organic thin film is coated as a lower layer so as to flatten the substrate surface. After an inorganic film such as a silicon oxide film is deposited as an intermediate layer, a photoresist is coated as an upper layer, and the upper photoresist is formed into a pattern by a normal photolithographic technique.
After the three-layered resist is formed as described above, the intermediate layer is etched using the upper photoresist pattern as a mask by anisotropic etching such as RIE using a gas containing a halogen as an etching gas, and the organic thin film serving as the lower layer is etched using a gas containing oxygen, thereby transferring the upper photoresist pattern to the two lower thin films.
However, the method of etching an Al film using a multilayered resist generally has complicated processes, and poses a problem of high cost. In addition, since this etching has a high aspect ratio, a microloading effect occurs when the Al film serving as a film to be processed is etched, and problems such as a high dependency of the etching rate on the pattern line width are posed.
In addition, the method of etching an Al film using a multilayered resist poses the following problems.
In a normal mask peeling process, although a photoresist is dissolved using an aqueous sulfuric acid solution or an aqueous hydrogen peroxide solution or removed by a plasma ashing method using an oxygen gas in accordance with the material of the underlying layer, only an organic thin film can be removed by this mask peeling process. Since the mask formed by the multilayered resist includes an inorganic thin film such as a silicon oxide film or the like, the mask is an inorganic mask. For this reason, the mask formed using the multilayered resist cannot be removed by the normal mask peeling means described above. Although an inorganic mask can be removed using a gas or the like containing a halogen such as fluorine or chlorine, in this case, a silicon oxide film, silicon layer, Al layer, and the like of a target substrate serving as underlying layers are also damaged.
That is, when a thin film is etched using a conventional RIE technique, in particular, when an Al thin film containing a small amount of Cu or the like is etched to form wiring layers, etching residues are formed due to the presence of Cu or the like, and reliability of the wiring layers is degraded. Although formation of the etching residues can be suppressed by high ion energy or the use of a multilayered resist, in this case, the following new problems are posed. That is, the etching selection ratio with respect to the resist is lowered, the processes are complicated, and the cost increases.